A transmission frequency used in a radar apparatus is determined in accordance with the application purpose. In general, when a high resolution or accuracy is required, a high frequency is used. A weather radar apparatus uses, as its transmission frequency, S band, C band or X band in the subdivisions of frequency. The frequencies are assigned in a predetermined frequency step such as 2.5 MHz based on the licensing system. However, if a self station and another station, which use different transmission frequencies, are installed at close locations, radio interference may occur between signals received by the two stations. If a signal from another radar site or the like is mixed into a received signal as an interference wave, the sensitivity of the received signal lowers, and the received power or Doppler velocity cannot accurately be measured.
Note that as a known reference associated with the present application, Jpn. Pat. Appln. KOKAI Publication No. 2002-139565 is disclosed.